Today, base stations are used for communicating with different kinds of user equipment, for example cell phones and laptops. A base station normally comprises a number of sector antennas, where each sector antenna is constituted by a vertically arranged column of radiating antenna, elements. Often, each antenna element is dual polarized, the two polarizations being mutually orthogonal.
A dual polarized sector antenna according to the above thus has two antenna ports, one for each polarization. The ports are coupled to at least one transmitter receiver unit, TRU, also called transceiver unit. A TRU commonly comprises one transmitter sub-unit, TX, and two receiver sub-units, RXA, RXB. The two receiver sub-units are normally used for two-way receiver diversity, i.e. each receiver sub-unit receives signals from one of two antenna ports, respectively, and the signals from each receiver sub-unit are then combined to get a maximized quality of the received signal.
If the receiver sub-units are connected to the two ports of a dual polarized antenna, one commonly refers to this as polarization diversity, and if the receiver sub-units are connected to two antennas that are separated by some distance, one refers to this as space diversity.
Preferably, it is desirable to have more than one transceiver per sector (or cell) in order to increase the capacity in the sector. FIG. 1a shows an example where two TRU's P1, P2 are connected to one sector antenna P3. In this case, a transmitter sub-unit P4 of the first TRU P1 is connected to a first antenna port P5 via a first duplex filter DPX1, while a transmitter sub-unit P6 of the second TRU P2 is connected to a second antenna port P7 via a second duplex filter DPX2. A respective first receiver sub-unit P8, P9 of the TRU's is connected to the first antenna port P5 via the first duplex filter DPX1. A respective second receiver sub-unit P10, P11 of the TRU's is connected to the second antenna port P7 via the second duplex filter DPX2.
If more than two TRU's are desired in one sector, while only having two antenna ports, it is necessary to combine the transmitters of the TRU's using for instance a hybrid combiner.
FIG. 1b illustrates an example where four TRU's P12, P13, P14, P15 are connected to one sector antenna P3 having two antenna ports P5, P7 as in the first case. In this case, the transmitter P16 of the first TRU P12 and the transmitter P17 of the second TRU P13 are combined in a first hybrid combiner P18A and then connected to the first antenna port P5 via a first duplex filter DPX1, while the transmitter P19 of the third TRU P14 and the transmitter P20 of the fourth TRU P15 are combined in a second hybrid combiner P18B and then connected to the second antenna port P7 via a second duplex filter DPX2. In a similar way as for the arrangement according to FIG. 1a, the respective first receiver sub-units P21, P22, P23, P24 of all TRU's are connected to the first antenna port P5 via the first duplex filter DPX1, and the respective second receiver sub-units P25, P26, P27, P28 of all TRU's are connected to the second antenna port P7 via the second duplex filter DPX2.
The drawback of using the hybrid combiners P18A, P18B is that half the output power is lost in each combiner.
Another possibility to accomplish four TRU's per sector is to have an additional dual polarized sector antenna, as shown in prior art FIG. 1c. Here, a first dual polarized sector antenna P29 and a second dual polarized sector antenna P30 having first antenna ports P31, P33 and second antenna ports P32, P34, respectively, are placed beside each other with a certain distance between the antennas P29, P30. The antenna and radio arrangement comprises four TRU's P35, P36, P37, P38. The first antenna's second antenna port P32 and the second antenna's first antenna port P33 are each connected to four receiver sub-units P39, P40, P41, P42; P43, P44, P45, P46 and one transmitter sub-unit P47, P48, via a respective duplex filter, DPX1, DPX2, while the first antenna's first antenna port P31 and the second antenna's second antenna port P34 only are connected to a respective transmitter sub-unit P49, P50 via a respective band pass filter, TXBP1, TXBP2. Obviously there are other ways to connect the TRU's to the antennas giving the same functionality.
This antenna and radio arrangement has a certain coverage which in many cases preferably is increased by improving the link budget in downlink as well as uplink.
The downlink budget can be improved by having transmitters with higher output power. One way to increase the output power is to let the transmitters of two TRU's operate according to TCC (Transmitter Coherent Combining), as described for example in WO 03/088511, where both the transmitters are provided with the same stream of modulating digital signals and operate at the same radio carrier frequency and the two amplified output signals are combined into one by means of a proper phase compensation in order to have the signals adding in phase. By means of this arrangement the output power is approximately doubled, and thereby the downlink is improved by 3 dB, but the downlink signal capacity is reduced to half since the two transmitters act as one. An improvement of 3 dB or more in uplink budget can be obtained by making use of the two pairs of receiver in the two TRU's to operate as a 4WRD (4-Way Receiver Diversity) receiver instead of two separate receivers with 2WRD (2-way Receiver Diversity), i.e. each of the four receivers receives signal from one unique antenna port and the signals from each receiver are combined to get a maximized quality of the received signal. The uplink signal capacity is then reduced to half since the two pairs of receivers in each TRU act as one 4WRD receiver.
An example of a TCC and 4WRD arrangement is shown in prior art FIG. 2, where a first dual polarized sector antenna P29 and a second dual polarized sector antenna P30 having a first antenna port P31, P33 and a second antenna port P32, P34, respectively, are placed beside each other with a certain distance between the antennas P29, P30 in the same way as described for FIG. 1c. 
The antenna and radio arrangement comprises four TRU's P51, P52, P53, P54. The first antenna's second antenna port P32 is connected to a first duplex filter DPX1, from which the antenna port P32 is connected to a respective second RX sub-unit P55, P56 in the first TRU P51 and the third TRU P53, and to a first combiner/hybrid P57. The first combiner P57 is connected to a respective TX sub-unit P58, P59 in the first TRU P51 and the second TRU P52, thus combining said TX sub-units P58, P59 to the first antenna's second antenna port P32. This pair of TX sub-units P58, P59 operates as one transmitter unit according to TCC.
In a similar way, the second antenna's first antenna port P33 is connected to a second duplex filter DPX2, from which the antenna port P33 is connected to a respective first RX sub-unit P60, P61 in the second TRU P52 and the fourth TRU P54, and to a second combiner/hybrid P62. The second power combiner/hybrid P62 is connected to a respective TX sub-unit P63, P64 in the third TRU P53 and the fourth TRU P54, thus combining said TX sub-units P63, P64 to the second antenna's first antenna port P33. These two TX sub-units P63, P64 operate as one transmitter unit according to TCC.
The remaining antenna ports P31, P34 are connected to two RX sub-units P65, P66; P67, P68 each, the first RX sub-units P65, P66 in the first TRU P51 and the third TRU P53, and the second RX sub-units P67, P68 in the second TRU P52 and the fourth TRU P54, via a respective band pass filter RXBP1, RXBP2.
The RX sub-units P65, P55, P61, P67 of the first TRU P51 and the second TRU P52 each receive signals from the four antenna ports P31, P32, P33, P34, respectively. This first set of four RX sub-units P65, P55, P61, P67 forms a first 4WRD receiver. Furthermore, the RX sub-units P66, P56, P60, P68 of the third TRU P53 and the fourth TRU P54 each receive signals from the four antenna ports P31, P32, P33, P34, respectively. This second set of four RX sub-units forms a second 4WRD receiver.
The arrangement according to FIG. 2 improves the downlink and the uplink budgets by approximately 3 dB. However, the signal capacity in both down- and uplink is reduced with a factor two, since the four TRU's P51, P52, P53, P54 operate in two pairs, where each pair transmits and receives the same digital signals.
There evidently exists a need for an enhanced antenna and radio arrangement where the down- and uplink budgets are improved and thereby coverage is increased at a lower cost, an extended coverage is achieved at the same cost, or a possibility to achieve an adaptive coverage using existent equipment. The object of the present invention is thus to provide such an enhanced antenna arrangement.